For a thermoelectric conversion element, an element providing the Peltier effect or the Seebeck effect is used. In recent years, use of this thermoelectric conversion element in a wide range is attracting people's attention because the structure is simple and the handling is easy so that stable characteristics can be maintained. In particular, as an electronic cooling element among the thermoelectric conversion elements, studies in a wide range are made towards optoelectronics, isothermalizing semiconductor lasers, and the like because local cooling and precision temperature control around room temperature can be made.
Referring to FIG. 17, the electronic cooling element described above or a thermoelectric module used for thermoelectric power generation is constructed in such a manner that p-type element (p-type thermoelectric conversion material) 5 and n-type element (n-type thermoelectric conversion material) 6 are joined via joining electrode (metal electrode) 7 to form a p-n element pair, and a plurality of the p-n element pairs are arranged in series. Current introduction terminals 8 and 9 are connected to joining electrodes 7 at the two ends of the series arrangement, respectively. Joining electrodes 7 are further sandwiched from the outside by a pair of ceramic substrates 10. At this time, the thermoelectric module is constructed in such a manner that one end of p-type element 5 and n-type elements 6 are heated and the other ends are cooled depending on a direction of the electric current that flows through a joining part. To this thermoelectric module, the heat flows in a direction of arrow H.
As the material for p-type element 5 and n-type element 6, the material having a large performance index Z is used.
[Math. 1]
A crystal material generally used as the material of p-type element 5 and n-type elements 6 is a Bi2Te3 material; however, the crystal material has a considerable cleavage property, and it is known in the art that, after passing through a slicing and dicing step or the like for obtaining a thermoelectric element from an ingot, there is raised a problem of an extremely low yield because of breakage or cracking.
In order to solve the problem, a method of manufacturing a thermoelectric conversion element module is attempted, the method comprising: heating step of mixing material powders to have a desired composition and heating and fusing the mixture; solidifying step of forming a solid-solution ingot of thermoelectric semiconductor materials having a rhombohedron structure (hexagonal crystal structure); a crushing step of crushing the solid-solution ingot to form a solid-solution powder; granulating step of uniformizing the particle size of the solid-solution powder; sintering step of pressurizing and sintering the solid-solution powder having a uniformized particle size; and hot thrashing forging step of performing hot plastic deformation for flatting the sintered powder body, thereby the crystal grains of the sintered powder structure is made to be oriented in a crystal orientation so as to obtain the excellent performance index (See, for example, Patent Literature 1).
Also, as a conventional method of manufacturing a thermoelectric conversion element module, there is known a manufacturing step including the steps of: producing an alloy ingot; crushing the alloy ingot in a vacuum or inert gas atmosphere containing an oxygen concentration of 100 ppm or less to obtain a source powder having an average particle size of 0.1 micrometers or more and less than 1 micrometer; and sintering the source powder by resistance heating while applying a pressure. The sintering step of the method comprises applying a pulsing electric current to the source powder for sintering by the Joule heat, and applying a pressure of 100 kg/cm2 or more and 1,000 kg/cm2 or less (9.8 to 98 MPa) to the source powder during the sintering. Through this sintering step, a particle made of a thermoelectric conversion material having a fine crystal size and having an excellent in process-ability is proposed (See, for example, Patent Literature 2).
Also, there is known a method of manufacturing a thermoelectric conversion element in which a melt of each of n-type and p-type semiconductors is sucked up into a fine quartz or glass pipe and solidifying the melt as it is, followed by cutting into a predetermined length to obtain a rod-shaped element (See, for example, Patent Literature 3).